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Sealing or blocking rear ports

Discussion in 'Amps and Cabs [BG]' started by Gman, Apr 15, 2002.


  1. Gman

    Gman

    Jan 4, 2000
    Indianapolis, IN
    I bought one of the Avatar 1-15 cabs that have been the topic of a few posts here. And I have a question for you speaker experts.

    The cab rocks. I really don't have any problem with its sound , but the rear ports kinda cramped my style at a show the other night. I had to set up in front of a wall with a bunch of stuff hanging on it...an old wall that was probably no more than paneling over studs. Well, you can probably figure out already what happened. I didn't notice a problem, until the beginning of "Blood and Roses" by The Smithereens. Intro is just bass.:eek: It sounded like the worst damned noise I've ever heard. The wall and everything on it was rattling.

    It looks like we'll be playing there more, and will be in the same place all the time.(After we move the pool table, but that's a story for another forum).

    So my question is, can I seal, block or plug those rear ports without damaging the Kappa Driver. If it will be like that again, I won't be able to use it. I was embarrased. It wasn't my rig, but it sure would have sounded like it to somebody that didn't know better.

    What do you think ?

    Dave
     
  2. Here's how you do it.

    1) Determine the internal volume of your cabinet.
    2) Determine the exact Eminence model installed.
    3) Download my spreadsheet
    4) Find the "Qts" and "Fs" values for your driver

    Link to Sealed Box Calculator

    Plug in the Qts and Fs numbers, and your cabinet volume. This will give you an idea of how it will operate. A Qtc between 0.577 and 0.707 is ok, and a bit higher will give you a mid-bass hump, but that might be perceived as "thumpy" bass.

    Don't expect to drive your cab hard below the F3 cutoff point. Sealed boxes have to move the cone MUCH farther than ported boxes to get the same amount of noise. You will notice the bass is tighter, though.

    It would be easier to move your cab away from the wall...

    :D
     
  3. Both my SWR cabs are rear ported, and if in a similar situation, I just stuff a towel or something in the port. I dont know about the theory of it, but it improves a bad situation. Works for me.
     
  4. Rockbobmel

    Rockbobmel Supporting Member

    At your local True Value you should find some pipe plugs for about $4. The kind that hs a giant wing nut and compresses a rubber sleeve outwards to seal. Be careful that you dont insert the plug too deep or you will crack the port. You can only go as deep as the plywood thickness is.

    I'm not stating that this is right or wrong to do, but only a way to try it. I tried it on some Acme B-2s, and it did not make a difference in bass throw.
     
  5. You end up changing alot of the speaker and cab design going from ported to sealed. To solve this same problem with my rig I bought some black pipe from the hardware store and made an l shaped piece that would allow me to "rotate" the air either to the side or even better straight up the back of the cab. Ultimately this is the reason I bought front firing cabs later on.

    The pipe job should cost you less then 20 dollars and its really easy to do.
     
  6. beermonkey

    beermonkey

    Sep 26, 2001
    Seattle, WA
    I'm also guessing the reason the wall was shuttering like a mofo is not because of the port on the back of your cab... bass guitars tend to produce low frequencies. Low frequencies tend to shake stuff, especially stuff that is flimsy.
     
  7. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Yes. And as low notes spread omnidirectional it does not make a difference if the ports are in the front or in the rear as long as the distance to the wall isn't TOO short (approx. 1ft.)
     
  8. Lovin Bass

    Lovin Bass

    Feb 8, 2002
    Los Angeles
    If nothing else, you could cut new ports in the front baffle board and seal up the rear ports. I don't know about the technical effects this would do so it is just a suggestion. Good Luck.